Boots of the aforedescribed type have been provided for hydraulic piston-and-cylinder units heretofore in such manner that the generally cylindrical segment of the boot, sometimes referred to as a bellows or cuff even though it need not have accordion-like folds, is directly connected to the widening segment in the region of the upper end of the cylinder, i.e. the end from which the piston rod emerges.
The widening segment in this case extends away from the piston rod at a comparatively small acute angle so that any transition between the widening segment and the cylindrical segment is relatively narrow and can come under excessive stress.
As the piston rod is extended, i.e. extends increasingly from the cylinder during the piston stroke, the angle may become even more acute.
In conventional constructions of such boots, practically the entire volume of the widening region is filled with gas to form a gas cushion with spring-like characteristics. This construction has the drawback that upon retraction of the piston rod and movement of the piston in the cylinder in a direction so as to retract the piston rod, the widening region of the boot is contracted in the form of a rolled membrane and the reduced volume in the interior of the boot must be compensated by a radial yielding thereof. This radial yielding is generally undefined, i.e. the boot distorts in nonuniform and uncontrolled fashion as the result of the presence of the gas cushion.
The outer contours of the boot may then deviate sharply from a cylinder.
This is especially disadvantageous when the device is used in an artificial joint or joint prosthesis for a human subject since, with such prostheses, the available space is limited. The uncontrolled bulging can result in rubbing and rupture of the membrane, rendering the motion-damping unit ineffective.